Study Notes: What’s This Thing Called ‘Spectroscopy’?

Spectroscopy is to do with the light an object absorbs or emits and its
relationship to the make-up of the object.

Atoms, ions and molecules emit or absorb light at certain wavelengths
that are unique to the particular atom, ion or molecule. Examination of
the light coming from an object can enable a picture to be built up of
the components that make up the object. Spectroscopy, for instance, has
been harnessed by astronomers to determine the composition of the objects
in the universe as well as other valuable information.

Astronomers pass the light from a star through a spectrograph which
is an instrument that separates the light into its various wavelengths,
producing a characteristic spectrum. This process is similar to the
way white light can be divided into its component colours (ie wavelengths)
upon travelling through a prism. By examining the spectrum for the
absence or presence of certain wavelengths, astronomers can determine
which chemical elements are present in the star as well as information
such as its temperature and density.

Spectroscopy in chemical analysis works in the same way. Put very simply,
light is shone on a sample and the light that leaves the sample is examined
to determine chemical composition or concentration.

Analytical chemists commonly measure absorbance (the amount of light
‘soaked up’) or, alternatively, transmittance (the amount
of light that passes straight through). Light that is absorbed and subsequently
emitted can also be measured and is known as emission.

Light covering a wide range of wavelengths (or energies) can be passed
through a sample. When we know the wavelengths that have been absorbed,
we know what chemical entities are present in the sample. If the sample
is radiated with a wavelength that is not absorbed, nothing will have
been learnt.

Spectroscopy involves specific wavelengths. The
energy associated with the wavelengths corresponds to the atoms, ions
or molecules in the sample moving to certain excited states. More
on this subject later.

That different atoms, ions and molecules absorb light at different wavelengths
of the electromagnetic spectrum illustrates the various forms of spectroscopy.
For example, there is infrared (IR) and ultraviolet (UV) spectroscopy.

In essence, spectroscopy concerns the study of the interaction of electromagnetic
radiation with atoms, ions and molecules. Spectroscopic instruments in
analytical chemistry use the information from these interactions to identify
components in a sample and measure their concentrations.